1. Field of the Invention
This invention relates generally to the field of optical beam steering systems, and more specifically to an apparatus and method for two dimensional optical beam steering for use in laser based radar systems.
2. Background of the Invention
Optical Phased Array (OPA) technology represents an enabling technology that makes possible simple, affordable, and lightweight laser beam steering with very precise stabilization, random access pointing, and programmable multiple simultaneous beams. Military applications include laser weapons, search and rescue, missile tracking, etc. Commercial side applications include laser printing, optical computing, etc. For all of the stated applications, an OPA technology is desired that exhibits high speed and large area scanning of laser beams covering wavelengths from the infrared to the ultraviolet. During the last decade, intensive efforts have been made to improve OPA performance and to develop OPAs based on nematic liquid crystals, active and passive optical waveguides using polymers, GaAs/AlGaAs, Silicon, Lithium Niobate, etc.
Liquid crystal (LC) is commonly used because of low driving voltage, large birefringence, and low cost fabrication techniques. Therefore, by applying appropriate driving voltages, the desired output phase pattern can be obtained and a deflection of the beam can be performed.
A static deflector device for deflecting a polarized infrared beam is suggested by Huignard et al. [U.S. Pat. No. 4,639,091, issued Jan. 27, 1987]. The static deflector consists of a liquid crystal layer and a modulator. Application of a staircase voltage waveform across the liquid crystal layer sandwiched between an optically transparent top striped electrode pattern and a reflective bottom electrode sets a diffraction grating of variable period inside the liquid crystal due to local variation of refractive index inside the liquid crystal layer. Huignard et al. address the two dimensional scanning via suggesting an apparatus for two beam deflectors by arranging the control electrodes 90° with respect to each other.
A two-dimensional transceiver utilizing four one beam deflectors, two for each of the orthogonal polarizations of the transceiver channels, a polarization rotator, and a polarization transformer is suggested by Lipchak et al. [U.S. Pat. No. 5,126,869]. The transmit and receive signals are on two linear and orthogonally polarized beams.
Although 2-D steering can be obtained for the liquid-crystal system by cascading two orthogonal oriented 1-D arrays, the efficiency, scanning speed, and scanning angle are still very low. Larger steering angles can be achieved by using a set of binary birefringent prisms. One polarization is deflected by the prism in one direction and the other polarization is deflected by the prism in the other direction. By rotating polarization between prism layers, binary beam steering at large angles can be accomplished. However, such systems are bulky and the polarizations of the intermediate birefringent prisms need to be rotated precisely for accurate angular control.
Optical waveguide based systems are capable of exhibiting large steering angles. The waveguides are arranged in an array with the required spacing. Both active and passive waveguides can be controlled to generate optical phase shift. For waveguide arrays made of Silicon (Hosseini A, Kwong D, Zhao Y, Chen Y-S, and Chen R. T, “Unequally-spaced Waveguide Arrays for Silicon Nano-membrane-based Efficient Large Angle Optical Beam Steering,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 15, No. 5, 1439-1446, 2009), additional heaters are placed alongside the waveguides in order to induce optical phase shift via the thermo-optic effect. Recently, GHz steering speed phased array optical beam steering has been reported for an OPA using an array of GaAs/AlGaAs active waveguides (Jarrahi M, Pease R. F. W, Miller D. A. B, and Lee T. H, “Optical switching based on high-speed phased array optical beam steering,” Applied Physics Letters. Vol. 92, 014106, 2008), but only about six degree steering angle is achieved. However, for waveguide based systems, it is also a very difficult task to arrange a two-dimensional OPA, as achieving high stacking efficiency of multiple layers is extremely challenging.
Designs are needed in the art to implement two-dimensional optical phased arrays with high speed and large angle tunability of beams with substantial relaxation in the fabrication difficulties associated with such structures.